The Collaborative International Dictionary of English v.0.48:

Phytology \Phy*tol"o*gy\ (f[imac]*t[o^]l"[-o]*j[y^]), n. [Phyto-
   + -logy: cf. F. phytologie.]
   The science of plants; a description of the kinds and
   properties of plants; botany. --Sir T. Browne.
   [1913 Webster] Phytomer

WordNet (r) 3.0 (2006):

phytology
    n 1: the branch of biology that studies plants [syn: botany,
         phytology]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "phytology":
   algology, aquiculture, botany, bryology, dendrology, fungology,
   hydroponics, mycology, paleobotany, phycology,
   physiological botany, phytobiology, phytochemistry, phytoecology,
   phytogeography, phytography, phytomorphology, phytonomy,
   phytopaleontology, phytopathology, phytotaxonomy, phytoteratology,
   phytotomy, phytotopography, plant anatomy, plant pathology,
   plant physiology, pomology, structural botany, systematic botany